Himesh Patel went busking in Norfolk in disguise to get in the "mindset" of a struggling musician for 'Yesterday'.
The British actor plays singer/songwriter Jack Malik in a world where The Beatles never existed in the movie and the composer Daniel Pemberton has revealed they sent him out dressed in a wig to play substandard songs to get him ready for the role.
Daniel told The Sun newspaper's Bizarre column: "We sent him busking which is one of my favourite things. We said, 'You've got to see what it's like to be a struggling singer-songwriter'.
"He played songs that we helped write for him.
"Not many were that good but he had to play them, just to get into the mindset of what it was like.
"He went into Galston, which is where a lot of it is filmed in Norfolk. They recorded it with secret cameras as well."
The film - which also features Ed Sheeran and Lily James - has a very special scene where Jack eventually comes face-to-face with the iconic band's late frontman John Lennon, played by an actor - who has not been named out of respect for the 'Imagine' hitmaker's legacy.
Opening up on the touching scene, former 'EastEnders' star Himesh admitted:"It kind of took my breath away, to be honest. It felt so poignant, of course, and yeah, really beautiful.
"This whole film is kind of a 'what if?' story, and it felt like the tip of that, you know? "Maybe there's a universe in which that's where John is. And then he still turns out to be this wonderful, thoughtful, philosophical man."
Director Danny Boyle recently revealed they didn't hold auditions for the special part.
He explained: "We just talked about this actor and I sent it to him. I knew him, and I knew he was a lifelong Lennon fan. For him, it was a special journey.
"He didn't want to be credited in the cast, in the list, and he didn't want it to be talked about or become a thing or anything like that.
"He just wanted to do it to honour someone that was very important in his imaginative life. He never met him or anything like that, but he played a huge part in his imaginative life."
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